WO1996036300A1

WO1996036300A1 - Prosthesis for joint replacement

Info

Publication number: WO1996036300A1
Application number: PCT/AU1996/000300
Authority: WO
Inventors: Ronald David Conry
Original assignee: Casttikulm Research Pty Ltd
Priority date: 1995-05-18
Filing date: 1996-05-17
Publication date: 1996-11-21
Also published as: AUPN306295A0

Abstract

A prosthesis has a stem (14) which is inserted into a socket (14) in a bone (15) of a recipient. The stem is formed with an inner end section (16) and a hollow outer end section (19) defined by a wall (17). A rod (22) extends from the inner end section (16) through the hollow section (19), and the rod is movable transversely relative to the wall (17). A ball (26) or other prosthetic formation is located on the outer end of the rod (22).

Description

Prosthesis for Joint Replacement

FIELD OF THE INVENTION

This invention relates to improvements in prosthesis and relates particularly to an improved prosthesis associated with joint replacement. However, the principles of the invention may be used with other forms of prosthesis.

A prosthesis for a typical joint replacement, such as a hip replacement, includes a stem which is implanted within the bone material of the recipient. The stem carries a ball on one end constituting part of the replacement hip. A bore or socket is formed in the bone and the stem is secured therein by suitable means while bone tissue is allowed to grow into special formations provided on the surface of the implant for that purpose. Implants are generally formed of a metal, such as titanium or titanium alloy, although they may be formed of any other castable or machinable material of sufficient strength and which is biologically compatible. BACKGROUND OF THE INVENTION It has been found that prosthesis implants tend to become loose within the formed bore or socket over a period of time notwithstanding the growth of bone material into the surface formations. Such looseness is believed to result from the inherent flexibility of bone material surrounding the inflexible implant. Relative movement which occurs between the implant and the surrounding bone may cause the bone material which has grown into the implant surface formations to be fractured and the implant therefore becomes painful to the recipient.

It is therefore desirable to provide an improved form of prosthesis which avoids the disadvantages associated with the relatively inflexible forms of prosthesis currently used. It is also desirable to provide an improved prosthesis which is able to be inserted within a socket in a bone and be retained firmly therein whilst permitting some flexibility between an operative part of the prosthesis and the bone structure.

It is also desirable to provide an improved prosthesis which is economical to manufacture and is relatively simple to position. SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a prosthesis a prosthesis comprising a stem to be received within a bore or socket formed in a bone of a recipient, the stem having an inner end section with a wall extending therefrom to define a hollow, outer end part, and a rod integral with the inner end section and passing through the hollow part, the rod having an outer end section which is movable laterally relative to the hollow part, and filter means in said hollow part engaged with the rod, the filter means being movable, flexible or resilient.

In one preferred form, the prosthesis is cast from titanium or a titanium alloy. However, other suitable materials which are known in the art and which are biologically compatible and which provides the necessary flexibility for the outer end portion on the rod to move relative to the outer end part of the stem.

The filler means may comprise any suitable, biologically compatible filler, such as a synthetic plastic material, or a natural material such as body fluids. The filler may be used simply to fill the hollow outer end part of the stem whilst permitting flexing movement of the outer end of the rod, or it may assist in absorbing or transmitting flexible movement of the rod to the walls of the outer end part of the stem.

A further feature of the present invention is the provision of either a formed surface discontinuity or a plurality of holes into which or through which the surrounding bone can grow when the stem is inserted within a socket formed in the bone of the recipient. Surface discontinuities may be formed on the wall of the hollow outer end of part of the stem. Such discontinuities may be effected either on the surface of the inner end section or by the use of sintered beads or by roughening the surface using shot or sand blasting techniques. These are known in the art and do not need to be described in greater detail. Alternatively, the surface may be etched or eroded using an electrical spark erosion technique so as to provide a surface formation into which the bone of the recipient can grow. The roughening provides a key for the bone growth to thereby assist in locking the prosthesis in the socket in the bone. The surface roughening may be formed by appropriate casting during manufacture of the prosthesis. Alternatively, and more preferably, the wall of the hollow end of the stem may be formed during casting with a plurality of holes or openings through which the bone can grow to lock the stem in position in the socket.

A still further feature of the present invention is the method of forming the stem of the invention by casting such that the combination of the shape of the stem and the casting method results in crystal growth in the longitudinal direction of the stem from the inner end section upwardly through the hollow, outer end part and simultaneously through the rod. By having the crystal growth extending longitudinally, the interconnection of the rod and the inner end section is free of stress and crystal boundary layer discontinuity thus ensuring the outer end of the rod is able to move laterally without any weakness developing at the interconnection.

In order that the invention is more readily understood, one embodiment thereof will now be described with reference to the accompanying drawings. DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional, elevational view of one form of prosthesis in accordance with the present invention;

Fig. 2 is a side elevational view of the prosthesis of Fig. 1 ;

Fig. 3 is a cross-sectional view similar to Fig. 1 showing the prosthesis in use; and Fig. 4 is an enlarged view of part of the wall of the prosthesis.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to the drawings, the prosthesis described herein comprises one part of a hip replacement prosthesis. It will be appreciated, however, that the invention is not so limited and the features of the invention may be utilized in any form of prosthesis which is to be implanted in or connected to bone tissue.

The prosthesis illustrated in Fig. 1 comprises a stem 12 which is adapted to be received within a socket or bore 14 formed in the bone 15 of a recipient. The socket or bore 14 may be formed by any suitable method such as boring with an appropriately shaped tool to provide a socket of the size and shape to closely receive the stem 12.

The stem comprises an elongated inner end section 16 which is preferably of solid material, although hollows, holes or other formations may be made therein or thereon to facilitate or permit bone growth which will assist in locking the stem in place.

A wall 17 extends from the inner end section 16 to define a hollow, outer end part. The wall 17 includes a flange portion 18 at its outer end which substantially closes the hollow part 19. The flange portion 18 is provided with an opening 21.

A rod 22 also extends from the inner end section 16. The rod 22 passes through the hollow part 19 and through the opening 21 in the flange portion 18. In the preferred embodiment illustrated, the rod 22 is integral with the inner end section 16 and the wall 17. It will be appreciated, however, that the rod may be formed as a separate part which is fastened to the inner end section 16 by securing means such as adhesive, welding, screw threads, pins or the like.

The hollow part 19 of the stem 12 is filled with a flexible, biologically compatible filling material 23. In one form of the invention this material may be a flexible or resilient synthetic plastic material or other synthetic material which will allow relative movement between the outer end 24 of the rod 22 and the wall 17 and flange portion 18 of the stem 12. In some instances of use of the invention, the filling material may comprise natural material such as body fluids. The outer end 24 of the rod 22 is formed with an appropriate ball 26, or other formation adapted for its intended prosthesis use, in a known manner.

As particularly shown in Fig. 3, in use, the stem 12 is engaged within the socket 14 and is held therein by an adhesive or by screws (not shown) or by other suitable means known in the art. The rod 22 is of dimensions as to permit lateral movement between the outer end part 24 thereof and the wall 17 of the hollow part 19 of the stem 12. Such relative movement or flexing of the rod 22 reduces the forces which would otherwise be transferred directly to the boundary between the prosthesis and the bone material is thus reducing the tendency of the bone material

15 flexing in use such as could loosen the engagement of the stem 12 within the socket 14. The flexing of the rod 22 therefore ensures that the inner end section

16 and wall 17 are secured retained within the socket 14 during the life of the prosthesis and any forces which might otherwise give rise to flexible movement of the bone are dissipated through the flexible rod 22. The filling material 23 may also act to dissipate lateral forces on the rod 22.

It will be seen that the prosthesis of the invention provides a flexible rather than a rigid interconnection between the ball 26 and the bone 15.

Referring to Fig. 4, the wall 17 has a section 27 formed with a plurality of holes 28 through which bone material can grow when the stem 12 is located within the socket 14. The holes 28 are formed of a shape which will allow bone material to grow around the material of the section 27 to thereby lock the stem 12 in the socket 14.

The prosthesis of the invention is preferably formed of titanium or a titanium alloy. However, any other material which is biologically compatible and is of a strength compatible with the intended use of the prosthesis may be used. Preferably, the material of the prosthesis is castable, and the stem is formed by casting and directional solidification in such a way that the crystal growth during the casting process proceeds longitudinally to produce an integrated inner end section 16, rod 22 and wall 17 with only minimal, if any, crystal boundary discontinuation in the region where the rod joins with the inner end section. This method of casting provides maximum strength and flexibility.

Claims

CLAIMS:

1. A prosthesis comprising a stem to be received within a bore or socket formed in a bone of a recipient, the stem having an inner end section with a wall extending therefrom to define a hollow, outer end part, and a rod integral with the inner end section and passing through the hollow part, the rod having an outer end section which is movable laterally relative to the hollow part, and filter means in said hollow part engaged with the rod, the filter means being movable, flexible or resilient.

2. A prosthesis as claimed in claim 1 wherein said inner end section is elongated.

3. A prosthesis as claimed in claim 1 or claim 2 wherein said wall has a flange portion at its outer end defining an opening through which the rod projects.

4. A prosthesis as claimed in any one of claims 1 to 3 wherein said rod carries a ball or other co-operating prosthetic-engaging means.

5. A prosthesis as claimed in any one of claims 1 to 4 wherein said inner end section, said wall and said rod are formed by casting as a single, integral unit, the casting being directionally solidified so that crystal growth of the cast material extends longitudinally.

6. A prosthesis as claimed in any one of claims 1 to 5 wherein said filter is a biologically compatible synthetic plastic material.

7. A prosthesis as claimed in any one of claims 1 to 6 wherein said filter means completely fills said hollow, outer end part.

8. A prosthesis as claimed in any one of claims 1 to 6 wherein a surface of the inner end section and/or a surface of the wall of the hollow, outer end part, is formed with a roughened area into which bone or the recipient can grow to lock the stem in its socket.

9. A prosthesis as claimed in any one of claims 1 to 6 wherein the wall of the hollow, outer end part is formed with a plurality of holes into which bone of the recipient can grow to lock the stem in its socket.

10. A prosthesis substantially as hereinbefore described with reference to the accompanying drawings.